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United States Patent |
5,335,503
|
Lee
|
August 9, 1994
|
Cooling method and apparatus
Abstract
A method and apparatus for cooling a heat load in which a pressure vessel
is provided to contain a cryogen as a saturated liquid and saturated vapor
separated by liquid-vapor interface. The pressure vessel has an inlet
adapted to receive a subcooled liquid cryogen and thereby convert it to a
saturated liquid through contact with the saturated vapor. A heated
overflow tube projects into the pressure vessel and is level, at one end,
with the liquid-vapor interface so that any increase in the saturated
liquid overflows into overflow tube and is vaporized to form the saturated
vapor. The heat transfer and the reintroduction of the vaporized liquid
can be effected through forced circulation provided by an ejector.
Preferably the pressure vessel is provided with two outlets for
discharging the saturated cryogen as saturated liquid or vapor. One or
more ejectors having high pressure inlet(s) in communication with the two
outlets of the pressure vessel are provided for drawing the heat transfer
fluid, after having cooled the heat load, into a heat transfer
relationship with the overflowed saturated liquid and then into mixture(s)
with the saturated liquid or saturated vapor, or both and for discharging
the same out of a high pressure outlet of the ejector to the heat load.
Inventors:
|
Lee; Ron C. (Bloomsbury, NJ)
|
Assignee:
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The BOC Group, Inc. (New Providence, NJ)
|
Appl. No.:
|
024713 |
Filed:
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March 1, 1993 |
Current U.S. Class: |
62/50.1; 62/50.3; 62/62; 62/64; 62/239 |
Intern'l Class: |
F25D 017/02; F25D 025/00 |
Field of Search: |
62/49.2,50.1,50.2,50.3,50.5,62,64,239
|
References Cited
U.S. Patent Documents
3163022 | Dec., 1964 | Hottenroth | 62/168.
|
3307366 | Mar., 1967 | Smith | 62/5.
|
3385073 | May., 1968 | Snelling | 62/45.
|
3447334 | Jun., 1969 | Kimball | 62/64.
|
3464222 | Sep., 1969 | Gramse | 62/45.
|
3570262 | Mar., 1971 | Gramse | 62/89.
|
4060400 | Nov., 1977 | Williams | 62/162.
|
4075869 | Feb., 1978 | Fitsall | 62/374.
|
4498306 | Feb., 1985 | Tyree, Jr. | 62/239.
|
4576010 | Mar., 1986 | Windecker | 62/64.
|
4726195 | Feb., 1988 | Klee | 62/62.
|
4768356 | Sep., 1988 | Volker | 62/50.
|
5018358 | May., 1991 | Lee et al. | 62/48.
|
Foreign Patent Documents |
3008355 | Sep., 1981 | DE | .
|
1581958 | Jul., 1990 | SU | .
|
Primary Examiner: Bennet; Henry A.
Assistant Examiner: Kilner; Christopher
Attorney, Agent or Firm: Rosenblum; David M., Cassett; Larry R.
Parent Case Text
REFERENCE TO RELATED CASE
This is a Continuation-in-Part of application Ser. No. 07/896,701, filed
Jun. 10, 1992, now abandoned.
Claims
I claim:
1. A method of circulating a heat transfer fluid along a circulation path
to cool a heat load, said method comprising:
containing a cryogen within a pressure vessel as a saturated liquid and as
a saturated vapor separated from one another by a liquid-vapor interface;
maintaining the liquid-vapor interface at a predetermined level by
withdrawing excess amounts of the saturated liquid from the pressure
vessel, vaporizing the excess amounts of the saturated liquid and
thereafter reintroducing the vaporized saturated liquid into the pressure
vessel;
introducing the cryogen, at a temperature no greater than the saturation
temperature of the cryogen, into the pressure vessel;
discharging the cryogen from the pressure vessel in a form of a least one
of the saturated liquid and the saturated vapor;
transferring heat from the heat transfer fluid to the cryogen after
discharge thereof; and
circulating the heat transfer fluid through the circulation path so that
the heat transfer fluid cools the heat load and is thereby heated and
thereafter, is cooled by the cryogen discharged from the pressure vessel;
the excess amounts of the saturated liquid being vaporized by transferring
further heat from the heat transfer fluid to the saturated liquid prior to
the heat transfer from the heat transfer fluid to the cryogen and after
the heat transfer fluid has been heated by the heat load;
the circulation of the heat transfer fluid at least promoted by converting
a portion of the total available thermodynamic energy of the saturated
cryogen, after discharge thereof, to circulation work.
2. The method of claim 1, wherein:
the cryogen upon its introduction into the pressure vessel is in a
subcooled state; and
the saturated liquid is created by passing the liquid form of the cryogen
through the saturated vapor to cause a condensation of a portion of the
saturated vapor.
3. The method of claim 1, wherein the heat transfer fluid comprises the
cryogen.
4. The method of claim 1 or 3, wherein both the saturated liquid and the
saturated vapor are discharged at predetermined rates in order to control
the cooling potential and the degree of circulation work provided by the
cryogen.
5. An apparatus for circulating a heat transfer fluid along a circulation
path to cool a heat load, said apparatus comprising:
a pressure vessel adapted to contain a cryogen as a saturated liquid and as
a saturated vapor separated from one another by a liquid-vapor interface;
the pressure vessel having, an inlet for introducing the cryogen into the
pressure vessel at a temperature of no greater than the temperature of the
cryogen, a heated overflow tube projecting into the pressure so that an
excess amount of the saturated liquid overflows into the heated overflow
tube and is vaporized to form the saturated vapor; and discharge means for
discharging the cryogen in a form of at least one of the liquid and the
vapor;
means connected to the discharge means and converting at least a portion of
the total available thermodynamic energy of the saturated cryogen, after
discharge thereof, to circulation work for circulating the heat transfer
fluid in the circulation path so that the heat transfer fluid cools the
heat load and is thereby heated and for cooling the heat transfer fluid
with the cyrogen discharged from the pressure vessel; and
a sealed pipe connected to the overflow tube so that the heat transfer
fluid, after having been heated by the heat load, transfers heat to the
over flow tube prior to the cooling of the heat transfer fluid with the
cryogen discharged from the pressure vessel.
6. The apparatus of claim 5, wherein:
the heat transfer fluid circulation and cooling means comprises at least
one venturi-type device having,
a high pressure inlet for receiving the cryogen discharged from the
discharge means,
a low pressure inlet, and
a high pressure outlet; and
the low pressure inlet and the high pressure outlet of the at least one
venturi-type device adapted to be operatively associated with the
circulation path so that the cryogen is discharged from the high pressure
outlet to the heat load and is thereby heated to cool the heat load and
thereafter, a portion of the cryogen is drawn into the low pressure inlet
to mix with the cryogen discharged from the discharge means.
7. The apparatus of claim 6, wherein a conduit is connected to the low
pressure inlet of the venturi-type device so that the cryogen, after
having been heated by the heat load, transfers heat to the overflow tube
prior to forming the mixture.
8. The apparatus of claim 6, wherein:
the discharge means comprises,
the pressure vessel having two outlets positioned such that one of the two
outlets is operable to discharge the saturated liquid and the other of the
two outlets is operable for discharging the saturated vapor, and
control means connected to the two outlets for selectively and individually
controlling the flow rates of the saturated liquid and vapor discharged
from the pressure vessel; and
the at least one venturi-type device is connected to the control means.
9. The apparatus of claim 8, wherein the heating means comprises, a sealed
pipe connected to the overflow tube, and a conduit connected to the low
pressure inlet of the at least one venturi-type ejector so that the
cryogen, after having been heated by the heat load, transfers heat to the
overflow tube prior to forming the mixture.
10. An apparatus for circulating a heat transfer fluid along a circulation
path to cool a heat load, said apparatus comprising:
a pressure vessel adapted to contain a cryogen as a saturated liquid and as
a saturated vapor separated from one another by a liquid-vapor interface;
the pressure vessel having, an inlet for introducing the cryogen into the
pressure vessel at a temperature of no greater than the saturation
temperature of the cryogen, an overflow tube projecting into the pressure
vessel so that excess amounts of the saturated liquid overflow into the
overflow tube and, discharge means for discharging the cryogen in a form
of at least one of the liquid and the vapor;
means connected to the discharge means and converting at least a portion of
the total available thermodynamic energy of the saturated cryogen, after
discharge thereof, to circulation work for circulating the heat transfer
fluid in the circulation path so that the heat transfer fluid cools the
heat load and is thereby heated and for cooling the heat transfer fluid
with the cryogen discharged from the pressure vessel; and
a heat exchanger with at least one pass connected to the overflow tube, the
heat exchanger positioned and configured to transfer further heat from the
heat transfer fluid to the excess amounts of the saturated liquid that
have overflowed into the overflow tube and thereby cool the heat transfer
fluid prior to cooling with the cryogen discharged from the pressure
vessel; and
means for drawing the excess amounts of the saturated liquid from the
overflow tube and through the heat exchanger and for reintroducing the
excess amounts of the saturated liquid into the pressure vessel.
11. The apparatus of claim 10, wherein the saturated cryogen drawing and
introducing means comprises a venturi-type device having, a set of high
and low pressure inlets and a high pressure outlet, the venturi-type
device connected to the pressure vessel so that the cryogen flows through
the high pressure inlet and is discharged through the high pressure outlet
into the inlet of the pressure vessel, the venturi-type device also
connected to the heat exchanger so that the excess amount of the cryogen
are drawn through the overflow tube, through the heat exchanger and then
into the low pressure inlet to mix with the cryogen prior to its entry
into the pressure vessel.
12. The apparatus of claim 11, wherein:
the heat transfer fluid comprises the cryogen discharged from the pressure
vessel;
the venturi-type device comprises a first venturi-type device, the set of
high and low pressure inlets and the high pressure outlet is a first set
of high and low pressure inlets and a first high pressure outlet; and
the heat transfer fluid circulation and cooling means comprises a second
venturi-type device having, a second set of high and low pressure inlets
and a second high pressure outlet, the second venturi-type device
connected to the discharge means of the pressure vessel so that cryogen
discharged from the discharge means flows to the high pressure inlet of
the second set of high and low pressure inlets, the second high pressure
outlet and the low pressure inlet adapted to be operatively associated
with the heat load so that the cryogen is discharged from the high
pressure outlet to the heat load and is thereby heated to cool the heat
load and thereafter, a portion of the cryogen is drawn into the low
pressure inlet to mix with the cryogen discharged from the discharge
means.
13. The apparatus of claim 12, further comprising a conduit connected to
the second venturi-type device and in communication with the low pressure
inlet of the second set of high and low pressure inlets so that the
portion of the cryogen flows through the conduit and into the low pressure
inlet of the second set thereof.
14. The apparatus of claim 10, wherein:
the discharge means comprises,
the pressure vessel having two outlets positioned such that one of the two
outlets is operable to discharge the saturated liquid and the other of the
two outlets is operable for discharging the saturated vapor, and
control means connected to the two outlets for selectively and individually
controlling the flow rates of the saturated liquid and vapor discharged
from the pressure vessel; and
the the heat transfer fluid circulation and cooling means is connected to
the control means.
15. The apparatus of claims 8 or 14, wherein the control means comprises
two proportional valves.
Description
BACKGROUND OF THE PRIOR ART
The present invention provides a method and apparatus for circulating a
heat transfer fluid along a circulation path, by forced circulation, to
cool a heat load. More particularly, the present invention relates to such
a method and apparatus in which a cryogen, in preferably a subcooled
liquid form, is converted into a saturated liquid through contact with a
saturated gaseous form of the cryogen, a portion of the total available
thermodynamic energy of either the saturated liquid, the saturated vapor,
or both, is utilized to at least promote circulation of the heat transfer
fluid and the heat transfer fluid is cooled by the saturated liquid and/or
the saturated vapor.
The prior art has provided a variety of cooling methods and apparatus in
which a cryogen, such as solid or liquid carbon dioxide, liquid nitrogen,
etc., is utilized to cool a heat load and to promote circulation of a heat
transfer fluid which can comprise evolved cryogenic vapor or a mixture of
cryogenic vapor and air to and from a heat load. An example of such an
apparatus is found in U.S. Pat. No. 3,163,022 in which the heat load
comprises perishables contained within an insulated refrigerated
compartment. An insulated refrigerant compartment is connected to the
refrigerated compartment by supply and return conduits. The refrigerant
compartment has a heat exchanger containing dry ice and a nozzle
projecting from the heat exchanger into a venturi-type ejector provided
within the supply conduit. The dry ice sublimates into a gas and the gas
is expelled into the ejector of the supply conduit and then into the
refrigerated compartment to cool the perishables. After having been heated
through the cooling of the perishables, the gas returns to the refrigerant
compartment through the return conduit. The returning gas transfers heat
to the dry ice through the heat exchanger and thereafter, mixes with the
sublimated gas in the ejector. The ejector produces a low pressure region
to draw the returning gas from the refrigerated compartment and past the
heat exchanger in the refrigerant compartment. Thus, a portion of the
total available thermodynamic energy of the sublimated gas, that is a sum
of its enthalpy and its kinetic energy, is being made to perform work in
forcing the circulation of the sublimated gas between the refrigerant and
refrigerated chambers. At the same time, the cooling potential of the
sublimated gas is being used to cool the perishables.
The amount of cooling and circulation are coupled due to the
self-pressurizing aspect of the apparatus. Therefore, the degree of
cooling and the amount of circulation are necessarily limited. As will be
discussed, the present invention provides a cooling method and apparatus
in which the cooling supplied and the circulation of the coolant can be
independently controlled over a greater range of possible operating
conditions than such a prior art device as disclosed in the '022 patent.
SUMMARY OF THE INVENTION
The present invention provides a method of circulating a heat transfer
fluid along a circulation path to cool a heat load. In accordance with
such method, a cryogen is contained within a pressure vessel as a
saturated liquid and as a saturated vapor separated from one another by a
liquid-vapor interface. The liquid-vapor interface is maintained at the
predetermined level. The cryogen is introduced into the pressure vessel at
a temperature of no greater than the saturation temperature of the cryogen
and the cryogen is discharged from the pressure vessel in a form of at
least one of the saturated liquid and the saturated vapor. After discharge
of the cryogen from the pressure vessel, heat is transferred from the heat
transfer fluid to the cryogen and the heat transfer fluid is circulated
through the circulation path so that the heat transfer fluid cools the
heat load and is thereby heated and thereafter, is cooled by the cryogen
discharged from the pressure vessel. The circulation of the heat transfer
fluid is at least promoted by converting a portion of the total available
thermodynamic energy of the saturated cryogen, after discharge thereof, to
circulation work.
In another aspect, the present invention provides an apparatus for cooling
a heat load. The apparatus is provided with a pressure vessel which is
adapted to contain a cryogen as a saturated liquid and as a saturated
vapor separated from one another by a liquid-vapor interface. The pressure
vessel has an inlet for introducing the cryogen into the pressure vessel
at a temperature no greater than the saturation temperature of the
cryogen. A heated overflow tube projects into the pressure vessel so that
an excess amount of the saturated liquid overflows into the overflow tube
and is vaporized to form the saturated vapor. A discharge means is
provided for discharging saturated cryogen in a form of at least one of
the saturated liquid and the saturated vapor. A means is also provided for
heating the heated overflow tube. Additionally, a means is connected to
the discharge means that converts at least a portion of the total
available thermodynamic energy of the saturated cryogen, after discharge
thereof, to circulation work for circulating the heat transfer fluid in
the circulation path. In the circulation path, the heat transfer fluid
cools the heat load and is thereby heated.
In a further aspect, the present invention provides an apparatus for
circulating a heat transfer fluid along the circulation path to cool a
heat load which again utilizes a pressure vessel. The pressure vessel is
adapted to contain a cryogen as a saturated liquid and as a saturated
vapor separated from one another by a liquid-vapor interface. The pressure
vessel has an inlet for introducing the cryogen into the pressure vessel
at a temperature of no greater than the saturation temperature of the
cryogen and an overflow tube projecting into the pressure vessel so that
excess amounts of the saturated liquid overflow into the overflow tube.
Discharge means are provided for discharging the cryogen in a form of at
least one of the liquid and the vapor from the pressure vessel. A means is
connected to the discharge means that converts at least a portion of the
total available thermodynamic energy of the saturated cryogen, after
discharge thereof, to circulation work for circulating the heat transfer
fluid in the circulation path. In the circulation path, the heat transfer
fluid cools the heat load and is thereby heated. The aforementioned means
cool the heat transfer fluid with the cryogen discharged from the pressure
vessel. A heat exchanger is provided with at least one pass connected to
the overflow tube. The heat exchanger is positioned and configured to
transfer further heat from the heat transfer fluid to the excess amounts
of the saturated liquid that have overflowed into the overflow tube. As a
result, the heat transfer fluid is cooled prior to being cooled with the
cryogen discharged from the pressure vessel. A means is also provided for
drawing the excess amounts of the saturated liquid from the overflow tube
and through the heat exchanger and for reintroducing the excess amounts of
the saturated liquid into the pressure vessel.
The incoming cryogen can be any cryogen having a temperature no greater
than saturation temperature and in fact could be two phase flow.
Preferably though, the incoming cryogen comprises a subcooled liquid which
will be converted into a saturated liquid upon its contact with the
saturated vapor. The energy for the conversion comes from a corresponding
portion of the vapor condensing into the saturated liquid form. The
conversion causes the incoming subcooled liquid to undergo an increase in
enthalpy and therefore a corresponding increase in its ability to do the
work involved in circulating the heat transfer fluid. In an apparatus
employing a heated overflow tube, the heated overflow tube can be heated
by tranferring further heat from the heat transfer fluid to excess amounts
of the saturated liquid that have overflowed into the heated overflow
tube. The further heat transferred will convert the subcooled liquid into
the saturated cryogen independent of flow rate and without the use of any
additional control systems or other process adjustment techniques over a
wide range of operation. These foregoing aspects of the present invention
discussed above are important because they allow the actual cooling
potential supplied by the apparatus to be adjusted through adjustment of
the flow rate of the subcooled liquid. Additionally, the relative amount
of work (compared to cooling duty) that can be extracted from the
saturated cryogen can be adjusted by varying the source pressure of the
cryogen because the enthalpy of the saturated cryogen will be a function
of such pressure. The relative amount of work can also be controlled by
adjusting the ratio of the gas/liquid withdrawal. Thus, the cooling
potential supplied and the work extracted from the cryogen can be
independently predetermined in an apparatus in accordance with the present
invention.
It is important to note that the term "subcooled liquid cryogen" means any
cryogen in liquid form having a temperature below the saturation
temperature of the cryogen. Furthermore, "heat transfer fluid" as used
herein and in the claims can mean the cryogen itself. For instance, the
saturated cryogen, either in liquid form or gaseous form or a combination
of the both, can be circulated to and from a heat load and then be
recooled by mixing with saturated cryogen being discharged from the
pressure vessel. Alternatively, "heat transfer fluid" can be a mixture of
the cryogen, initially discharged as the saturated cryogen from the
pressure vessel, and another fluid such as air present within a
refrigerated container. Additionally, "heat transfer fluid" can be
completely distinct from the cryogen, for instance air circulating within
a refrigerated container that never comes into direct contact with the
cryogen. As will be discussed, the constituency of the "heat transfer
fluid" depends on the physical embodiment in which the present invention
is utilized. Furthermore, the term "total thermodynamic energy" of the
saturated cryogen means a sum of its enthalpy and its kinetic energy.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims distinctly pointing out the
subject matter that Applicant regards as his invention, it is believed
that the invention will be better understood when taken in conjunction
with the accompanying drawings in which:
FIG. 1 is a cross-sectional view of a cryogenic forced circulation cooling
apparatus in accordance with the present invention connected to a storage
vessel for storing a subcooled liquid cryogen; and
FIG. 2 is a fragmentary, cross-sectional schematic view of a cryogenic
forced circulation cooling apparatus in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the FIG. 1, an apparatus 10 in accordance with the
present invention is illustrated. Apparatus 10 is connected to a storage
vessel 12 which is specifically designed to contain liquid nitrogen in a
subcooled state. Storage vessel 12 is a pressure vessel and can be
designed to contain other cryogens such as liquid carbon dioxide, liquid
oxygen, liquid argon, etc. In this regard, the term "cryogen" will be
understood to mean, herein and in the claims, liquid nitrogen, carbon
dixoide, oxygen and argon.
As is common and known in the art, storage vessel 12 is the type of storage
vessel that has a pressure building and regulating circuit. As such, the
pressure of the liquid nitrogen delivered from storage vessel 12 can be
predetermined. Apparatus 10 is in turn connected to a refrigerated
compartment 14 which could be the trailer of a refrigerated truck or other
insulated container for storing perishables. Thus, refrigerated
compartment 14 is serving as a heat load to be cooled by apparatus 10. It
is worth noting, however, that this is exemplary only and as can be
appreciated, the present invention is useful for a variety of cooling
applications, for instance cooling applications involving the formation of
plastic articles.
Apparatus 10 is provided with a pressure vessel 16 which is adapted to
contain nitrogen as a saturated liquid 18 and a saturated vapor 20
separated by a liquid-vapor interface 22. Subcooled liquid nitrogen from
storage vessel 12 enters pressure vessel 16 through an inlet 24 thereof.
Upon entry, the subcooled liquid nitrogen is converted into saturated
liquid 18 as will be described more fully hereinbelow. A series of baffle
plates 25 are provided to ensure that entering subcooled liquid nitrogen
contacts saturated vapor 20.
It should be pointed out that the illustrative use of subcooled liquid
nitrogen is in no way meant to be a limitation on the scope of the present
invention. A saturated liquid cryogen, such as saturated liquid nitrogen
could be used with apparatus 10. Furthermore, a cryogen could enter
pressure vessel 16 under conditions of two phase flow.
An overflow tube 26 projects into pressure vessel 16 and acts to
predetermine the level of liquid-vapor interface 22 or in other words the
amounts of saturated liquid 18 or saturated vapor 20 that pressure vessel
16 will contain. As will be discussed, saturated liquid 18 and/or
saturated vapor 20 is discharged from pressure vessel 16 during use of
apparatus 10. The rate of discharge, relative to the rate at which
subcooled liquid nitrogen (or for that matter other possible cryogens at
other thermodynamic states) enters pressure vessel 16 causes an excess
amount of saturated liquid 18 to be produced. This excess amount of
saturated liquid nitrogen (saturated liquid 18) will cause saturated
liquid 18 to overflow into overflow tube 26. Thus, the level of
liquid-vapor interface 22 will remain constant. It should be pointed out
that if liquid nitrogen enters pressure vessel 16 under conditions of a
saturated two-phase flow, then vapor must be removed from pressure vessel
16 at a greater rate than vapor enters so that an excess amount of
saturated liquid is created to overflow into overflow tube 26.
As will be discussed, overflow tube 26 is heated to cause the saturated
liquid 18 therewithin to vaporize into saturated vapor 20. When overflow
tube 26 is heated, the total amount of heat transfer to saturated liquid
18 will be sufficient to just produce saturated vapor 20 because upon
vaporization of saturated liquid 18 within overflow tube 26, saturated
vapor 20 will climb overflow tube 26 and thus, essentially, will not
participate in any further heat transfer with the heat being transferred
to overflow tube 26. The amount of overflow will naturally be the sum of
saturated liquid 18 produced through condensation of saturated vapor 20
necessary to convert the incoming subcooled liquid, and any saturated
liquid 18 that overflows due to direct gas withdrawal. It is evident that
when only saturated liquid 18 is withdrawn, that the heat supplied to
overflow tube 26 is effectively transferred in a quantity just sufficient
to accomplish the subcooled liquid to saturated liquid conversion.
As illustrated, overflow tube 26, pressure vessel 16, and refrigerated
compartment 14 are covered by a layer of insulation 27 in a manner well
known in the art.
Pressure vessel 16 is also provided with an outlet 28 to discharge
saturated liquid 18 and outlet 30 to discharge saturated vapor 20. The
saturated liquid 18 is discharged to an ejector 32 and saturated vapor 20
is discharged to ejector 34. The nitrogen being discharged from pressure
vessel 16, or for that matter any other cryogen being utilized, serves as
the heat transfer fluid. As will be discussed, the heat transfer fluid can
be completely separate and distinct from the cryogen being utilized. In
the illustrated embodiment, however, the nitrogen will be discharged from
ejectors 32 and 34 into a distributor manifold 36 having distributor
nozzles 38. The nitrogen will then in turn be discharged from distributor
nozzles 38 onto the article(s) to be refrigerated. This causes heat to be
transferred from the article(s) back to the nitrogen. The thus heated
nitrogen is then drawn into ejectors 32 and 34 through a branched conduit
40 having two branches 40a and 40b to mix with saturated liquid 18 and
saturated vapor 20 being discharged from pressure vessel 16. The direction
of circulation or the circulation path is indicated by arrowhead line 41.
It is understood that ejectors 32 and 34 could be connected to
refrigerated container 14 or any other heat load to be cooled by supply
and return conduits.
More specifically, ejector 32 is provided with a liquid nozzle 42 connected
to outlet 28 via control valve 44. A gas nozzle 46 of ejector 34 is
connected to outlet 30 by way of a control valve 48. Control valves 44 and
48 are proportional valves to adjust the flow rates of saturated liquid 18
and saturated vapor 20 to ejectors 32 and 34. Liquid nozzle 42 and gas
nozzle 46 serve as high pressure inlets to ejectors 32 and 34,
respectively. Liquid nozzle 42 and gas nozzle 46 are designed in a manner
well known in the art to increase the velocities of saturated liquid 18 or
saturated vapor 20 and thereby create regions of low pressure within
mixing chambers 50 and 52 of ejectors 32 and 34, respectively. These low
pressure regions draw in the heated nitrogen, which in the illustrated
embodiment would be heated nitrogen vapor, into low pressure inlets 54 and
56 connected to branches 40a and 40b of branched conduit 40. Pressure is
recovered by diffuser sections 58 and 60 of ejectors 32 and 34. The
mixtures of heated nitrogen vapor and saturated liquid and vapor formed
within ejectors 32 and 34 are then discharged to the heat load through
distributor manifold 36 connected to diffuser sections 58 and 60 which
serve as high pressure outlets of ejectors 32 and 34. As can be
appreciated by those skilled in the art, in certain applications
distributor manifold 36 could be dispensed with.
A pipe 62, sealed at opposite ends, is connected to overflow tube 26 in a
T-like configuration to form a heat exchanger employed in transferring
further heat from the nitrogen, after having been heated by the heat load,
to overflow tube 26 and therefore saturated liquid contained within
overflow tube 26. To this end, pipe 62 is contained within branched
conduit 40. Alternatively, the present invention contemplates that
overflow tube 26 could be heated by a separate heating coil or other means
not involving the cryogen or other possible separate heat transfer fluid.
This however would not be preferred in that it would add a complexity not
present in the illustrated embodiment. Even more importantly, the heating
of overflow tube 26 as set forth in the illustrated embodiment conserves
cooling. The transfer of further heat from the heat transfer fluid
(nitrogen in the illustrated embodiment) cools the nitrogen which is mixed
or recycled back into the saturated nitrogen being discharged under
pressure from pressure vessel 16.
As can be appreciated, the cooling provided by either saturated liquid 18
and saturated vapor 20 or a mixture of the two can be determined
independently of their flow rates and without the use of any additional
control systems or other process adjustment techniques. Additionally, the
work potential of the saturated cryogen, again either saturated liquid 18,
saturated vapor 20, or a mixture thereof can be balanced with its cooling
capacity through adjustment of valves 44 and 48. As is known, saturated
vapor has more work potential due to its increased enthalpy. Thus, the use
of either saturated liquid 18 or saturated vapor or the mixture thereof
can determine work potential to be delivered. As mentioned above, the
relative amount of work as compared with cooling duty to be extracted from
the cryogen whether dispensed solely as a liquid or a gas or a mixture can
be additionally controlled through regulation of the outlet pressure of
storage vessel 12.
Many design variations are possible in accordance with the present
invention. For instance, pressure vessel 16 could be modified to have
single outlet, either 28 or 30, to utilize the total available energy and
cooling potential of either saturated liquid 18 or saturated vapor 20. In
such case, only ejector 32 or ejector 34, as appropriate, would be
employed. In this regard it is possible to supply a single ejector with
two outlets 28 and 30. In such case, provision would have to be made to
mix saturated liquid 18 and saturated vapor 20 either before or in the
single ejector utilized. For instance it is possible to produce a single
ejector having liquid and gas nozzles 42 and 46 incorporated into its
design. Moreover, a single outlet level with liquid-vapor interface 22
could be used in conjunction with a tube projecting into pressure vessel
16 and having a flexible section so that the tube can be raised into
saturated vapor 18 or lowered into saturated liquid 20 by an electrically
operated solenoid. Additionally, other venturi-type devices could be
substituted for ejectors 32 and 34 as are well known in the art. For
instance, another type of venturi-type device not illustrated herein but
known in the art consists of a device in which an annular nozzle is used
to induce high speed flow and a low pressure region. It should therefore
be noted that the term "venturi-type device" as used herein and in the
claims means any device in which a high pressure motive fluid, for
instance the saturated nitrogen discharged from pressure vessel 16
entrains and increases the pressure of an entrained fluid, for instance,
the nitrogen after having been heated by the heat load.
In the illustrated embodiment, circulation is produced by ejectors 32 and
34 discharging the nitrogen (comprising mixtures of the saturated vapor
and liquid 18 and 20 and recirculated heated nitrogen vapor having been
heated by the heat load) and then drawing the heated nitrogen back to
ejectors 32 and 34 after having cooled the heat load. Thus, a forced
circulation is set up within refrigerated compartment 14 by ejectors 32
and 34. Additionally, the heated nitrogen is then being cooled after
having been heated by mixing with saturated vapor and liquid 18 and 20. It
should be mentioned that an application of the present invention could
involve the addition of auxiliary fans to help the circulation. In such
case ejectors 32 and 34 would only help promote circulation.
Ejectors 32 and 34, or for that matter other venturi-type devices that
could be substituted therefor, utilize a portion of the total available
energy potential of the cryogen, either saturated liquid, saturated vapor
or both to produce or at least to promote circulation. Thus, a fluid
driven motor connected to a fan could serve the same purpose as ejectors
32 and 34, with of course different performance characteristics. In such
case nitrogen would be expelled from the fluid driven motor and the fan
would produce circulation of the nitrogen. Another possible embodiment
would include a heat exchange coil connected to the outlet of the fluid
driven motor and open to the atmosphere. In such embodiment, the heat
transfer fluid would comprise resident air contained within the
refrigerated container. The air would circulate to be cooled by the heat
exchange coil, warmed by the heat load, i.e. perishables, and then drawn
into the conduit surrounding the heat exchanger associated with the
overflow tube to heat subcooled liquid contained therewithin. As can be
appreciated, the air would never mix with the cryogen as in the
illustrated embodiment. Thus, the heat transfer fluid of such embodiment
would be completely distinct and separate from the cryogen being used to
cool the heat transfer fluid and to produce the circulation work involved
in circulating the air. In the illustrated embodiment, however, the heat
transfer fluid is the cryogen being added to refrigerated container 14 and
thus, a vent 60 is provided to vent excess cryogen.
FIG. 2 illustrates an alternative embodiment of an apparatus 100 in
accordance with the present invention. Apparatus 100 in use would be
connected to a storage vessel such as storage vessel 12. The liquid
cryogen enters apparatus 100 from the storage vessel through an entry line
102. Apparatus 100 could be employed within a refrigerated compartment
such as refrigerated compartment 14, in which case, arrowheads 104 and 106
(representing the intake and exhaust of heat transfer fluid) would lie at
the end and beginning of the circulation path. Alternatively, apparatus
100 could be used to deliver a cryogenic heat transfer fluid formed of
saturated cryogen to an external heat load. Although not illustrated,
apparatus 100, in the same manner as apparatus 10, would be encased within
insulation to minimize heat leakage.
Apparatus 100 is provided with a pressure vessel 108 which is adapted to
contain a cryogen such as nitrogen as a saturated liquid 110 and a
saturated vapor 112 separated by a liquid-vapor interface 114. Preferably,
subcooled liquid nitrogen enters pressure vessel 108 through an inlet 116
thereof. Upon entry, the subcooled liquid nitrogen is converted into
saturated liquid 110. A tray 118 is provided which forces the incoming
cryogen to travel in a thin layer along its surface and thereby into an
intimate contact with saturated vapor 112 to facilitate the condensing
operation.
In apparatus 100, as has been pointed out for apparatus 10, the
illustrative use of subcooled liquid nitrogen is in no way meant to be a
limitation on the present invention. Furthermore, a saturated liquid
cryogen could be used with apparatus 10 as well as a cryogen entering
pressure vessel 108 under conditions of two-phase flow. Overflow tube 120
projects into pressure vessel 108 to predetermine the level of
liquid-vapor interface 114. Excess amounts of saturated liquid 112 will
overflow into overflow tube 120. In apparatus 10, further heat is
transferred from the heat transfer fluid to the saturated liquid by way of
a heat exchanger, preferably a heat exchanger 122.
It is to be noted that a central advantage of apparatus 10 is its
self-regulating nature. That is, a sufficient amount of heat is introduced
into apparatus 10 to convert incoming subcooled liquid into saturated
liquid by vaporizing excess saturated liquid that has overflowed into the
overflow tube with the heat transfer fluid. This self regulation can also
be a disadvantage in that it can limit the range of operability of
Apparatus 10. For instance, if liquid cryogen is being supplied at too
fast a rate or gaseous cryogen is removed from pressure vessel at too fast
a rate, conditions of bi-directional flow will occur within overflow tube
26 which can cause it to choke up. Additionally, it is difficult to
optimize the design of heat exchanger 62 to take up little space. These
problems are by and large solved in apparatus 100 by providing an
operation in which flow of saturated liquid is forced through overflow
tube 120 and heat exchanger 122.
The conditions of forced flow are effected by means of a first ejector 124
of the same design as ejector 32. Ejector 124 is provided with a low
pressure inlet 126 and a high pressure inlet 128. Through a venturi
effect, saturated liquid and possibly an amount of entrained gas will be
drawn through heat exchanger 122 and then mixed with incoming liquid
cryogen to be discharged out of high pressure outlet 130 of first ejector
124 and into inlet 116 of pressure vessel 108. It is to be noted here that
the overflowing liquid in case of apparatus 100 will be fully vaporized
and in addition may be superheated within heat exchanger 122. In case of
no saturated gas withdrawal from pressure vessel 108, then heat exchanger
122 will exchange an amount of thermal energy to exactly effect the
conversion of subcooled liquid to saturated liquid without any active
control. This operation is similar to that of apparatus 10. Obviously, if
withdrawing gas, heat exchanger 122 must also exchange an amount of
thermal energy sufficient to supply the gas withdrawn over an amount of
gas that may be supplied through inlet 102. Simply stated, the energy
through the heat exchanger will automatically satisfy an energy balance
between the incoming and outgoing fluid streams of cryogen.
Saturated liquid and/or gas is withdrawn from pressure vessel 108 and is
then introduced into a second ejector 142 having a second set of low and
high pressure inlets 144 and 146, respectively, and a second high pressure
outlet 148. Control valves 150 and 152 control the process in the same
manner as control valves 44 and 48 of apparatus 10. Saturated cryogen
enters high pressure inlet 146 of second ejector 142 to draw heat transfer
fluid, for instance, nitrogen discharged from high pressure outlet 148 of
ejector 142 which has already been circulated through the heat transfer
path to cool the heat load. A conduit 159 can be provided to enclose heat
exchanger 122 and to conduct the heat transfer fluid in a heat transfer
relationship with heat exchanger 122 prior to entering low pressure inlet
144 of ejector 142.
Preferably, a level detector 160 is utilized in conjunction with the
present invention as well as valves 162 and 164. Level detector 160, fully
disclosed in U.S. Pat. No. 5,157,154, is employed to sense a low level of
saturated liquid 110 lying below the overflow tube. If saturated liquid
reaches such low level with valves 150 and 152 in a closed position, valve
162 would be commanded to open to replenish saturated liquid within
pressure vessel 108. Valve 164 would also be commanded to close as it
would in case of any detection of the low level of saturated liquid by
level detector 160 to prevent further gas generation. Although not
illustrated, but as would occur to one skilled in the art, the actuation
of valves 160 and 162 in response to the level sensed by level detector
160 and the sensing of position and also possibly the control of valves
150 and 152 would be controlled by a conventional controller. The
controller, not illustrated would be either an analog controller or a
programmable logic computer programmed to function in the above-referenced
manner.
As with apparatus 10, numerous modifications could be made to apparatus
100. For instance, pressure vessel 108 could be modified to have a single
outlet, either outlet 132 or outlet 140. Additionally, other venturi-type
devices could be substituted for ejectors 124 and 142. As with apparatus
10, auxiliary fans could be utilized to help circulation and/or a fluid
driven motor could serve in place of ejector 142. As stated previously,
apparatus 100 could be utilized within a refrigeration cabinet and as
such, a heat exchange coil could be connected to the outlet of a fluid
driven motor with the heat exchange coil open to the atmosphere. In such
case the heat transfer fluid would comprise resident air contained within
the refrigerated container.
While a preferred embodiment has been shown and described, as will occur to
one skilled in the art, numerous additions, omissions and changes can be
made without departing from the spirit and scope of the present invention.
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